Also:

 - don't mark a node as from the new generation if it is dirty, since we
   know it still has to be built.

 - establish the rule that you can't call setState() during initState()
   or build().

 - make syncChild() return early for unchanged children.

 - update the tests, including adding a new one.

Move the animation libraries into src/animation and change importers to use
package:sky/animation.dart. Also, move scheduler.dart into the animation
library so that the animation library can be self-contained.

Makes PageableList with itemsWrap:true work again.

Plumbed the itemsWrap parameter through to HomogenousViewport.

Fixes issue #877.

Convert Dismissable to use the ScrollStart, ScrollUpdate, and ScrollEnd gestures. Support for fling gestures is TBD.

Included a basic unit test that checks that one item can be dismissed with a press-drag-release gesture.

Fixed the scroll gesture recognizer: if the last pointer goes up and candidate recognizers still exist, then reject the gesture.

This parameter makes it easier to do math when you don't want to center your
tranform at (0, 0).

If there are no other gestures in the arena, we should kick off the scroll
gesture right away. This change pulled a refactoring of how we dispatch events
to Widgets. Now we dispatch events to Widgets interleaved with their associated
RenderObjects. (Previously we dispatched to all of the RenderObjects first.)

add/remove/add smoketest
basic vertical behaviour
basic horizontal behaviour

This patch replaces the scroll gestures used by Scrollable with ones detected
by the ScrollGestureDetector.

We're now using it at the widget layer for everything except scrolling and
flinging.

These gestures use the new gesture detection system.

Also includes changes to widget_tester sufficient to
test scrolling.

@abarth

Also, rename build_utils.dart to widget_tester.dart. These files are now named
for their most commonly used classes.

Finally, add a .analysis_options to silence the (intentional) analyzer warnings
in append_child_test.dart.

We can make all the test pass by removing more of the mocks and using the real
code.

Added some unit test infrasture for checking layers.

Most of this work in this patch is building out the test framework to the point
where we can write this test.

This lets clients listen for when a widget with a given global key goes through
a sync operation. We'll need this for mimic to track its mimicable when it
moves around the tree.